1. Field of Technology
This invention relates to a process for removing phenylacetylene contaminants from styrene, particularly relating to a selective hydrogenation method for phenylacetylene in the presence of C8 fraction, a byproduct in the process of ethylene cracking from petroleum hydrocarbon. This technical proposal can hydrogenate phenylacetylene in the fractions, whereas the content of styrene remains un-hydrogenated or is enhanced; hydrogenated C8 fraction produces minor amount of gum, and can be used as feedstock for petroleum hydrocarbon cracking C8 fraction in the process of styrene extraction.
2. Background
Ethylene cracked gasoline contains high level of aromatics, which is rich in styrene. A set of 1000 kt ethylene cracking apparatus can crack nearly 5% of styrene from gasoline and 40% of those from C8 fraction which is rich in styrene. In the prior art, C6-C8 fractions are directly undergone two-stage hydrogenation, hydrogenating and extracting unsaturated aromatics such as styrene. Eventually, styrene is recovered in the form of ethylbenzene in the presence of dimethylbenzene. Dimethylbenzene normally contains 40-60% of ethylbenzene, which will affect its yields if used directly as PX feedstock so as to impact cost-effectiveness. It will bring significant benefits if styrene can be extracted from C8 fraction. In recent years, the production of cracked gasoline is growing along with the increase of million ton of ethylene cracking plants establishing in China. Breakthrough technology for extracting and recycling styrene from cracked gasoline and its application in industry become the key and the focus point.
C8 fraction contains high level of styrene and minor amount of phenylacetylene. Phenylacetylene and styrene share similar chemical structure, therefore they have similar choices of extraction solvents. If styrene is extracted directly, phenylacetylene will be extracted along side. C8 fraction from ethylene cracked gasoline contains nearly 0.1˜1% of phenylacetylene. The purity and quality of styrene products are difficult to improve because of the presence of phenylacetylene. Therefore, phenylacetylene must be removed before styrene is extracted in order to obtain polymer-grade styrene products.
Presently, the best way to remove phenylacetylene from cracked gasoline is to selectively hydrogenate phenylacetylene and convert it into ethylbenzene or styrene.
Chinese patent CN1852877A discloses a method that involves reducing phenylacetylene contaminants in the presence of styrene monomers. A styrene monomer stream containing minor amount of phenylacetylene and a hydrogenation gas containing hydrogen are supplied to the hydrogenation reactor. The styrene monomer stream and the hydrogen are brought into contact with a catalyst bed containing a hydrogenation catalyst comprising a reduced copper compound on theta θ-alumina support. The hydrogenation reactor is operated at a temperature of at least 60° C. and a pressure of at least 30 psig to hydrogenate phenylacetylene to produce styrene. This technique features high reaction temperature, incomplete hydrogenation of phenylacetylene (70%), and high loss rate of styrene (nearly 3% styrene is converted into ethylbenzene).
Chinese patent CN101475438A discloses another selective hydrogenation method for phenylacetylene in the presence of styrene, mainly aiming to solve the problem of high loss rate of styrene in prior art. Said invention uses hydrocarbon fraction containing phenylacetylene as feedstock; reaction temperature is within the range of 15˜100° C.; weight space velocity is within the range of 0.01˜100 h−1; a molar ratio of hydrogen to phenylacetylene in the range of 1˜30:1; reaction pressure is within the range of −0.08˜5.0 Mpa. Under the condition, feedstock is brought to contact with a catalyst containing carbon oxide. Phenylacetylene in the reaction effluent is then hydrogenated to styrene. Said technique solves the problem of high loss rate of styrene. However, the hydrogenation of phenylacetylene is incomplete and generates gum after hydrogenating. Gum formation in feedstock leads to loss of styrene and consequently the polymers are left in the solvent during following process of styrene extraction and distillation, resulting in low extraction rate of solvent. In addition, the remained gum left at the surface of catalyst will gradually lower the catalyst performance and further deactivate catalyst, eventually shorten the catalyst lifetime.